The switching circuitry means are indispensable in various integrated circuit applications such as, for example, switched capacitor circuits for filters with reduced harmonic distortion in which the electronic switches realized with MOS transistors are driven by distinct timing signals without superimposed phases.
A rather simple type of electronic switch commonly used by those skilled in the art is the so-called `pass-transistor` consisting of two complementary CMOS transistors connected in mutual parallel with their respective source and drain terminals and driven by signals in phase opposition upon switching.
Such a switch often does not exhibit shortcomings in integrated circuits with sufficiently high power supply voltage on the order of at least 5 V.
But when the supply voltage is lower, typically around 3 V, as is the present trend for many integrated circuits, it is necessary to take in consideration what in the technical literature is known as `body` effect where the term `body` indicates the actual substrate of a transistor.
Indeed, in any CMOS process, one of two complementary transistors is provided in its own substrate called a well or tub.
For example, in an N-well CMOS process, it is the p-channel transistor which is provided in an independent substrate defined N-well and vice versa for the P-well CMOS process.
As known to those skilled in the art, the actual threshold voltage upon conduction of the transistors of an integrated circuit of the CMOS type depends on the `body` effect in accordance with which the actual threshold voltage VTH of a transistor increases in absolute value when the voltage VSB between source and body of the transistor is other than zero, i.e., if the transistor substrate is not short-circuited with the source region, and is the highest when the source region is insulated from the substrate.
For the best operation of a pass-transistor the n-channel transistor substrate is kept at the lowest potential existing in the integrated circuit while the substrate of the p-channel transistor is kept at the highest potential.
If the supply voltage is too low, for certain voltages values, at the two ends of the switch it may be impossible, due to the body effect, to switch to conduction at least one of the two transistors which constitute the pass-transistor and thus permit correct operation of the switch.
As explained in the article `Higher Sampling Rates in SC Circuits by On-Chip Clock-Voltage Multiplication` by F. Krummenacher, H. Pinier and A. Guillaume Laboratoire d'electronique general, EPFL, Lausanne, Switzerland--the solution used by those skilled in the art is that of driving the switch transistors with signals in `voltage-boosted` phase opposition, i.e. having amplitude almost doubled with respect to the value of the power supply voltage.
Correct operation of the switch is thus assured even with low supply voltages, but this solution involves the shortcoming of requiring a complicated circuit for generation of the voltage-boosted driving signals.